Summary Humans in every culture judge some actions and people as positive, others as negative. It is clearly beneficial to distinguish supportive and friendly individuals from harmful and malicious ones, and evolutionary biologists, comparative psychologists, and cultural anthropologists have argued that an ability to evaluate others is essential for navigating the social world. Studies proposed here will explore the developmental origins of social evaluation; specifically, how infants & toddlers (a) understand and predict how other individuals evaluate social actions and the agents who perform them; and (b) form their own positive and negative attitudes towards agents based on the actions that they engage in. Series 1 employs looking-time measures to investigate infants' assessment and understanding of several types of positive and negative interactions, including those involving reciprocation and retaliation. Series 2 examines infants' positive and negative evaluations of individuals based on their social actions, using infants' reaching behaviors as an indication of their assessments. Series 3 explores the extent to which young children's social evaluations are explicitly available, using verbal judgments and choices of whom to reward or punish; and explores how children's social evaluation of an individual influences their willingness to learn from that individual. The study of the development of social evaluation connects to disciplines such as social psychology, cultural anthropology, behavioral economics, and evolutionary theory. Results from these experiments will shed light both on central questions in developmental psychology, and on broader questions about the role of innate knowledge and cultural learning in the genesis of human social interaction.